Method and device for acquiring physiological data during tissue stimulation procedure

ABSTRACT

A method and system of providing therapy to a patient implanted with an array of electrodes is provided. A train of electrical stimulation pulses is conveyed within a stimulation timing channel between a group of the electrodes to stimulate neural tissue, thereby providing continuous therapy to the patient. Electrical parameter is sensed within a sensing timing channel using at least one of the electrodes, wherein the first stimulation timing channel and sensing timing channel are coordinated, such that the electrical parameter is sensed during the conveyance of the pulse train within time slots that do not temporally overlap any active phase of the stimulation pulses.

RELATED APPLICATION

The present application claims the benefit under 35 U.S.C. §119 to U.S.provisional patent application Ser. No. 61/221,987, filed Jun. 30, 2009.The foregoing application is hereby incorporated by reference into thepresent application in its entirety.

FIELD OF THE INVENTION

The present invention relates to tissue stimulation systems, and moreparticularly, to a system and method for sensing information during astimulation procedure.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, and theapplication of tissue stimulation has begun to expand to additionalapplications such as angina pectoralis and incontinence. Deep BrainStimulation (DBS) has also been applied therapeutically for well over adecade for the treatment of refractory chronic pain syndromes, and DBShas also recently been applied in additional areas such as movementdisorders and epilepsy. Further, in recent investigations PeripheralNerve Stimulation (PNS) systems have demonstrated efficacy in thetreatment of chronic pain syndromes and incontinence, and a number ofadditional applications are currently under investigation. Furthermore,Functional Electrical Stimulation (FES) systems such as the Freehandsystem by NeuroControl (Cleveland, Ohio) have been applied to restoresome functionality to paralyzed extremities in spinal cord injurypatients.

Each of these implantable neurostimulation systems typically includes anelectrode lead implanted at the desired stimulation site and animplantable pulse generator (IPG) implanted remotely from thestimulation site, but coupled either directly to the electrode lead orindirectly to the electrode lead via a lead extension. Thus, electricalpulses can be delivered from the neurostimulator to the stimulationelectrode(s) to stimulate or activate a volume of tissue in accordancewith a set of stimulation parameters and provide the desired efficacioustherapy to the patient. A typical stimulation parameter set may includethe electrodes that are sourcing (anodes) or returning (cathodes) thestimulation current at any given time, as well as the amplitude,duration, rate, and burst rate of the stimulation pulses.

The neurostimulation system may further comprise a handheld remotecontrol (RC) to remotely instruct the neurostimulator to generateelectrical stimulation pulses in accordance with selected stimulationparameters. The RC may, itself, be programmed by a technician attendingthe patient, for example, by using a Clinician's Programmer (CP), whichtypically includes a general purpose computer, such as a laptop, with aprogramming software package installed thereon.

The use of sensed electrical information, such as, e.g., impedance,field potential neural activity, etc., is of increasing importance inneurostimulation applications, such as SCS, which may some day provideintelligent and autonomous closed-loop control of the neurostimulation,thereby reducing the direct involvement of the patient in managing thetherapy. With respect to SCS, there are currently two limitations thatmake sensing of electrical parameter information to achieve on-the-flyclosed-loop stimulation control difficult to accomplish: (1) the sensingof the electrical parameter data information interferes with thedelivery of therapy (i.e., stimulation is halted); and (2) the samplingrate is not high enough to allow fast change detection and adaptation.

There, thus, remains a need for an improved method and system for moreefficiently sensing electrical parameter information while continuouslyproviding stimulation therapy to tissue.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, a method ofproviding therapy to a patient implanted with an array of electrodes.The method comprises conveying a train of electrical stimulation pulseswithin a stimulation timing channel between a group of the electrodes tostimulate neural tissue (e.g., spinal cord tissue), thereby providingcontinuous therapy to the patient. The method further comprises sensingan electrical parameter (e.g., an electrical impedance, a fieldpotential, and/or an evoked action potential) within a sensing timingchannel using at least one of the electrodes. The method may furthercomprise transmitting sub-threshold pulses within the sensing timingchannel between another group of the electrodes, wherein the electricalparameter is sensed in response to the transmission of the sub-thresholdpulses.

In either event, the first stimulation timing channel and sensing timingchannel are coordinated, such that the electrical parameter is sensedduring the conveyance of the pulse train within time slots that do nottemporally overlap any active phase of the stimulation pulses. Forexample, a hold-off period can be provided after the active phase of thestimulation pulses, such that no electrical parameter is sensed duringthe hold-off period.

One exemplary method further comprises conveying another train ofelectrical stimulation pulses within another stimulation timing channelbetween another group of the electrodes, thereby providing furthercontinuous therapy to the patient. In this case, the stimulation timingchannels and sensing timing channel are coordinated, such that theelectrical parameter is sensed during the conveyance of the pulse trainswithin time slots that do not temporally overlap any active phase of thestimulation pulses. Furthermore, the stimulation timing channels may becoordinated, such that the stimulation pulses within the respectivepulse trains do not temporally overlap any active phase of each other.The stimulation timing channels may be independently programmed withdifferent stimulation parameters.

In accordance with a second aspect of the present inventions, aneurostimulation system is provided. The neurostimulation systemcomprises analog output circuitry configured for conveying a train ofelectrical stimulation pulses within a stimulation timing channelbetween a selected group of an array of electrodes to stimulate neuraltissue in a manner that provides continuous therapy to the patient. Theneurostimulation system further comprises monitoring circuitryconfigured for sensing electrical parameter (e.g., an electricalimpedance, a field potential, and/or an evoked action potential) withina sensing timing channel using at least one of the electrodes. In oneembodiment, the analog output circuitry is configured for transmittingsub-threshold pulses within the sensing timing channel between anothergroup of electrodes, and the monitoring circuitry is configured forsensing the electrical parameter in response to the transmission of thesub-threshold pulses.

The neurostimulation system further comprises control circuitryconfigured for coordinating the first stimulation timing channel andsensing timing channel, such that the electrical parameter is sensedduring the conveyance of the pulse train within time slots that do nottemporally overlap any active phase of the stimulation pulses. Thecontrol circuitry may be configured for coordinating the firststimulation timing channel and sensing timing channel by providing ahold-off period after the active phase of the stimulation pulses, suchthat no electrical parameter is sensed during the hold-off period. Thecontrol circuitry may, e.g., take the form of a microcontroller or maytake the form of circuitry, such as a digital state machine.

In one embodiment, the analog output circuitry is further configured forconveying another train of electrical stimulation pulses within anotherstimulation timing channel between another group of the electrodes in amanner that provides further continuous therapy to the patient, and thecontrol circuitry is configured for coordinating the stimulation timingchannels and sensing timing channel, such that the electrical parameteris sensed during the conveyance of the pulse trains within time slotsthat do not temporally overlap any active phase of the stimulationpulses. In this case, the control circuitry may be further configuredfor coordinating the stimulation timing channels, such that thestimulation pulses within the respective pulse trains do not temporallyoverlap any active phase of each other. The neurostimulation system mayfurther comprise memory configured for storing different stimulationparameters for independently programming the stimulation timingchannels. The neurostimulation system may further comprise telemetrycircuitry configured wirelessly transmitting the sensed electricalparameter.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is plan view of one embodiment of a spinal cord stimulation (SCS)system arranged in accordance with the present inventions;

FIG. 2 is a plan view of the SCS system of FIG. 1 in use with a patient;

FIG. 3 is a profile view of an implantable pulse generator (IPG) used inthe SCS system of FIG. 1;

FIG. 4 is a plot of mono-phasic cathodic electrical stimulation energy;

FIG. 5a is a plot of bi-phasic electrical stimulation energy having acathodic stimulation pulse and an active recharge pulse;

FIG. 5b is a plot of bi-phasic electrical stimulation energy having acathodic stimulation pulse and a passive recharge pulse;

FIG. 6 is a block diagram of the internal components of the IPG of FIG.3;

FIG. 7 is a plot of stimulation pulses that are transmitted in an Nnumber of stimulation timing channels, and a sensed electrical parametermeasured within a sensing timing channel;

FIG. 8 is a plot of stimulation pulse trains transmitted within TimingChannels 1-3 and a sensed electrical parameter measured within TimingChannel 4; and

FIG. 9 is a plot of the stimulation pulse trains and a sensed electricalparameter of FIG. 8 after arbitration has been performed to preventoverlap of the stimulation pulses and a sensed electrical parameter.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The description that follows relates to a spinal cord stimulation (SCS)system. However, it is to be understood that the while the inventionlends itself well to applications in SCS, the invention, in its broadestaspects, may not be so limited. Rather, the invention may be used withany type of implantable electrical circuitry used to stimulate tissue.For example, the present invention may be used as part of a pacemaker, adefibrillator, a cochlear stimulator, a retinal stimulator, a stimulatorconfigured to produce coordinated limb movement, a cortical stimulator,a deep brain stimulator, peripheral nerve stimulator, microstimulator,or in any other neural stimulator configured to treat urinaryincontinence, sleep apnea, shoulder sublaxation, headache, etc.

Turning first to FIG. 1, an exemplary SCS system 10 generally includesone or more (in this case, two) implantable stimulation leads 12, animplantable pulse generator (IPG) 14, an external remote controller RC16, a clinician's programmer (CP) 18, an External Trial Stimulator (ETS)20, and an external charger 22.

The IPG 14 is physically connected via one or more percutaneous leadextensions 24 to the stimulation leads 12, which carry a plurality ofelectrodes 26 arranged in an array. In the illustrated embodiment, thestimulation leads 12 are percutaneous leads, and to this end, theelectrodes 26 are arranged in-line along the stimulation leads 12. Inalternative embodiments, the electrodes 26 may be arranged in atwo-dimensional pattern on a single paddle lead. As will be described infurther detail below, the IPG 14 includes pulse generation circuitrythat delivers electrical stimulation energy in the form of a pulsedelectrical waveform (i.e., a temporal series of electrical pulses) tothe electrode array 26 in accordance with a set of stimulationparameters.

The ETS 20 may also be physically connected via the percutaneous leadextensions 28 and external cable 30 to the stimulation leads 12. The ETS20, which has similar pulse generation circuitry as that of the IPG 14,also delivers electrical stimulation energy in the form of a pulsedelectrical waveform to the electrode array 26 in accordance with a setof stimulation parameters. The major difference between the ETS 20 andthe IPG 14 is that the ETS 20 is a non-implantable device that is usedon a trial basis after the stimulation leads 12 have been implanted andprior to implantation of the IPG 14, to test the responsiveness of thestimulation that is to be provided.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation leads 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation parameter sets. The IPG 14 mayalso be operated to modify the programmed stimulation parameters toactively control the characteristics of the electrical stimulationenergy output by the IPG 14.

The CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions. The CP 18 may perform this function by indirectlycommunicating with the IPG 14 or ETS 20, through the RC 16, via an IRcommunications link 36. Alternatively, the CP 18 may directlycommunicate with the IPG 14 or ETS 20 via an RF communications link (notshown). The clinician detailed stimulation parameters provided by the CP18 are also used to program the RC 16, so that the stimulationparameters can be subsequently modified by operation of the RC 16 in astand-alone mode (i.e., without the assistance of the CP 18). Theexternal charger 22 is a portable device used to transcutaneously chargethe IPG 14 via an inductive link 38. Once the IPG 14 has beenprogrammed, and its power source has been charged by the externalcharger 22 or otherwise replenished, the IPG 14 may function asprogrammed without the RC 16 or CP 18 being present.

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, andexternal charger 22 will not be described herein. Details of exemplaryembodiments of these devices are disclosed in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

As shown in FIG. 2, the electrode leads 12 are implanted within thespinal column 42 of a patient 40. The preferred placement of theelectrode leads 12 is adjacent, i.e., resting upon, the spinal cord areato be stimulated. Due to the lack of space near the location where theelectrode leads 12 exit the spinal column 42, the IPG 14 is generallyimplanted in a surgically-made pocket either in the abdomen or above thebuttocks. The IPG 14 may, of course, also be implanted in otherlocations of the patient's body. The lead extension 24 facilitateslocating the IPG 14 away from the exit point of the electrode leads 12.As there shown, the CP 18 communicates with the IPG 14 via the RC 16.

Referring now to FIG. 3, the external features of the stimulation leads12 and the IPG 14 will be briefly described. One of the stimulationleads 12(1) has eight electrodes 26 (labeled El -E8), and the otherstimulation lead 12(2) has eight electrodes 26 (labeled E9-E16). Theactual number and shape of leads and electrodes will, of course, varyaccording to the intended application. The IPG 14 comprises an outercase 44 for housing the electronic and other components (described infurther detail below), and a connector 46 to which the proximal ends ofthe stimulation leads 12(1) and 12(2) mate in a manner that electricallycouples the electrodes 26 to the electronics within the outer case 44.The outer case 44 is composed of an electrically conductive,biocompatible material, such as titanium, and forms a hermeticallysealed compartment wherein the internal electronics are protected fromthe body tissue and fluids. In some cases, the outer case 44 may serveas an electrode.

As will be described in further detail below, the IPG 14 includes pulsegeneration circuitry that provides electrical conditioning andstimulation energy in the form of a pulsed electrical waveform to theelectrode array 26 in accordance with a set of stimulation parametersprogrammed into the IPG 14. Such stimulation parameters may compriseelectrode combinations, which define the electrodes that are activatedas anodes (positive), cathodes (negative), and turned off (zero),percentage of stimulation energy assigned to each electrode(fractionalized electrode configurations), and electrical pulseparameters, which define the pulse amplitude (measured in milliamps orvolts depending on whether the IPG 14 supplies constant current orconstant voltage to the electrode array 26), pulse width (measured inmicroseconds), pulse rate (measured in pulses per second), and burstrate (measured as the stimulation on duration X and stimulation offduration Y).

Electrical stimulation will occur between two (or more) activatedelectrodes, one of which may be the IPG case. Simulation energy may betransmitted to the tissue in a monopolar or multipolar (e.g., bipolar,tripolar, etc.) fashion. Monopolar stimulation occurs when a selectedone of the lead electrodes 26 is activated along with the case of theIPG 14, so that stimulation energy is transmitted between the selectedelectrode 26 and case. Bipolar stimulation occurs when two of the leadelectrodes 26 are activated as anode and cathode, so that stimulationenergy is transmitted between the selected electrodes 26. For example,an electrode on one lead 12 may be activated as an anode at the sametime that an electrode on the same lead or another lead 12 is activatedas a cathode. Tripolar stimulation occurs when three of the leadelectrodes 26 are activated, two as anodes and the remaining one as acathode, or two as cathodes and the remaining one as an anode. Forexample, two electrodes on one lead 12 may be activated as anodes at thesame time that an electrode on another lead 12 is activated as acathode.

The stimulation energy may be delivered between a specified group ofelectrodes as monophasic electrical energy or multiphasic electricalenergy. As illustrated in FIG. 4, monophasic electrical energy includesa series of pulses that are either all negative (cathodic), oralternatively all positive (anodic). Multiphasic electrical energyincludes a series of pulses that alternate between positive andnegative.

For example, as illustrated in FIGS. 5a and 5b , multiphasic electricalenergy may include a series of biphasic pulses, with each biphasic pulseincluding a cathodic (negative) stimulation pulse (during a first phase)and an anodic (positive) recharge pulse (during a second phase) that isgenerated after the stimulation pulse to prevent direct current chargetransfer through the tissue, thereby avoiding electrode degradation andcell trauma. That is, charge is conveyed through the electrode-tissueinterface via current at an electrode during a stimulation period (thelength of the stimulation pulse), and then pulled back off theelectrode-tissue interface via an oppositely polarized current at thesame electrode during a recharge period (the length of the rechargepulse).

The second phase may have an active recharge pulse (FIG. 5a ), whereinelectrical current is actively conveyed through the electrode viacurrent or voltage sources, and a passive recharge pulse, or the secondphase may have a passive recharge pulse (FIG. 5b ), wherein electricalcurrent is passively conveyed through the electrode via redistributionof the charge flowing from coupling capacitances present in the circuit.Using active recharge, as opposed to passive recharge, allows fasterrecharge, while avoiding the charge imbalance that could otherwiseoccur. Another electrical pulse parameter in the form of an interphasecan define the time period between the pulses of the biphasic pulse(measured in microseconds).

Turning next to FIG. 6, the main internal components of the IPG 14 willnow be described. The IPG 14 includes analog output circuitry 50 capableof individually generating electrical stimulation pulses via capacitorsC1-C16 at the electrodes 26 (E1-E16) of specified amplitude undercontrol of control logic 52 over data bus 54. The duration of theelectrical stimulation (i.e., the width of the stimulation pulses), iscontrolled by the timer logic circuitry 56. The analog output circuitry50 may either comprise independently controlled current sources forproviding stimulation pulses of a specified and known amperage to orfrom the electrodes 26, or independently controlled voltage sources forproviding stimulation pulses of a specified and known voltage at theelectrodes 26.

Any of the N electrodes may be assigned to up to k possible groups or“channels.” In one embodiment, k may equal four. The channel identifieswhich electrodes are selected to synchronously source or sink current tocreate an electric field in the tissue to be stimulated. Amplitudes andpolarities of electrodes on a channel may vary, e.g., as controlled bythe RC 16. External programming software in the CP 18 is typically usedto set stimulation parameters including electrode polarity, amplitude,pulse rate and pulse width for the electrodes of a given channel, amongother possible programmable features.

The N programmable electrodes can be programmed to have a positive(sourcing current), negative (sinking current), or off (no current)polarity in any of the k channels. Moreover, each of the N electrodescan operate in a bipolar mode or multipolar mode, e.g., where two ormore electrode contacts are grouped to source/sink current at the sametime. Alternatively, each of the N electrodes can operate in a monopolarmode where, e.g., the electrode contacts associated with a channel areconfigured as cathodes (negative), and the case electrode (i.e., the IPGcase) is configured as an anode (positive).

Further, the amplitude of the current pulse being sourced or sunk to orfrom a given electrode may be programmed to one of several discretecurrent levels, e.g., between 0 to 10 mA in steps of 0.1 mA. Also, thepulse width of the current pulses is preferably adjustable in convenientincrements, e.g., from 0 to 1 milliseconds (ms) in increments of 10microseconds (μs). Similarly, the pulse rate is preferably adjustablewithin acceptable limits, e.g., from 0 to 1000 pulses per second (pps).Other programmable features can include slow start/end ramping, burststimulation cycling (on for X time, off for Y time), interphase, andopen or closed loop sensing modes.

The operation of this analog output circuitry 50, including alternativeembodiments of suitable output circuitry for performing the samefunction of generating stimulation pulses of a prescribed amplitude andwidth, is described more fully in U.S. Pat. Nos. 6,516,227 and6,993,384, which are expressly incorporated herein by reference.

The IPG 14 further comprises monitoring circuitry 58 for monitoring thestatus of various nodes or other points 60 throughout the IPG 14, e.g.,power supply voltages, temperature, battery voltage, and the like. Themonitoring circuitry 58 is also configured for measuring physiologicaldata, which may be used in a closed-loop or open-loop fashion with thetissue stimulation function.

For example, the impedance between the respective electrodes 26 and theIPG case 44 can be measured. Notably, the electrodes 26 fit snuglywithin the epidural space of the spinal column, and because the tissueis conductive, there is an impedance associated therewith that indicateshow easily current flows therethrough. Because implanted electricalstimulation systems depend upon the stability of the devices to be ableto convey electrical stimulation pulses of known energy to the targettissue to be excited, measuring electrode impedance is important inorder to determine the coupling efficiency between the respectiveelectrode 26 and the tissue.

For example, if the electrode impedance is too high, the respectiveelectrode 26 may be inefficiently coupled to the tissue that it is tostimulate. As a result, an excessive amount of compliance voltage mayneed to be generated in order to effectively supply stimulation energyto the electrode 26 if the analog output circuitry 50 usescurrent-controlled sources, thereby resulting in an inefficient use ofthe battery power, or the stimulation energy supplied to the electrode26 may be otherwise inadequate if the analog output circuitry 50 usesvoltage-controlled sources. Other electrical parameter data, such asfield potential and evoked action potential, may also be measured todetermine the coupling efficiency between the electrodes 26 and thetissue.

Measurement of the electrode impedance also facilitates fault detectionwith respect to the connection between the electrodes 26 and the analogoutput circuitry 50 of the IPG 14. For example, if the impedance is toohigh, that suggests the connector 42 and/or leads 12 may be open orbroken. If the impedance is too low, that suggests that there may be ashort circuit somewhere in the connector 42 and/or leads 12. In eitherevent (too high or too low impedance), the IPG 14 may be unable toperform its intended function.

The impedance measurement technique may be performed by measuringimpedance vectors, which can be defined as impedance values measuredbetween selected pairs of electrodes 26. The interelectrode impedancemay be determined in various ways. For example, a known current (in thecase where the analog output circuitry 50 is sourcing current) can beapplied between a pair of electrodes 26, a voltage between theelectrodes 26 can be measured, and an impedance between the electrodes26 can be calculated as a ratio of the measured voltage to knowncurrent. Or a known voltage (in the case where the analog outputcircuitry 50 is sourcing voltage) can be applied between a pair ofelectrodes 26, a current between the electrodes 26 can be measured, andan impedance between the electrodes 26 can be calculated as a ratio ofthe known voltage to measured current.

The field potential measurement technique may be performed by generatingan electrical field at selected ones of the electrodes 26 and recordingthe electrical field at other selected ones of the lead electrodes 26.This may be accomplished in one of a variety of manners. For example, anelectrical field may be generated conveying electrical energy to aselected one of the electrodes 26 and returning the electrical energy atthe IPG case 44. Alternatively, multipolar configurations (e.g., bipolaror tripolar) may be created between the lead electrodes 26. Or, anelectrode that is sutured (or otherwise permanently or temporarilyattached (e.g., an adhesive or gel-based electrode) anywhere on thepatient's body may be used in place of the case IPG outer case 44 orlead electrodes 26. In either case, while a selected one of theelectrodes 26 is activated to generate the electrical field, a selectedone of the electrodes 26 (different from the activated electrode) isoperated to record the voltage potential of the electrical field.

The evoked potential measurement technique may be performed bygenerating an electrical field at one of the electrodes 26, which isstrong enough to depolarize (or “stimulate”) the neurons adjacent thestimulating electrode beyond a threshold level, thereby inducing thefiring of action potentials (APs) that propagate along the neuralfibers. Such stimulation is preferably supra-threshold, but notuncomfortable. A suitable stimulation pulse for this purpose is, forexample, 4 mA for 200 μS. While a selected one of the electrodes 26 isactivated to generate the electrical field, a selected one or ones ofthe electrodes 26 (different from the activated electrode) is operatedto record a measurable deviation in the voltage caused by the evokedpotential due to the stimulation pulse at the stimulating electrode.

Further details discussing the measurement of electrical parameter data,such as electrode impedance, field potential, and evoked actionpotentials, as well as other parameter data, such as pressure,translucence, reflectance and pH (which can alternatively be used), todetermine the coupling efficiency between an electrode and tissue areset forth in U.S. patent application Ser. No. 10/364,436, entitled“Neural Stimulation System Providing Auto Adjustment of Stimulus Outputas a Function of Sensed Impedance,” U.S. patent application Ser. No.10/364,434, entitled “Neural Stimulation System Providing AutoAdjustment of Stimulus Output as a Function of Sensed Pressure Changes,”U.S. Pat. No. 6,993,384, entitled “Apparatus and Method for Determiningthe Relative Position and Orientation of Neurostimulation Leads,” andU.S. patent application Ser. No. 11/096,483, entitled “Apparatus andMethods for Detecting Migration of Neurostimulation Leads,” which areexpressly incorporated herein by reference.

The IPG 14 further comprises processing circuitry in the form of amicrocontroller (μC) 62 that controls the control logic over data bus64, and obtains status data from the monitoring circuitry 58 via databus 66. The IPG 14 additionally controls the timer logic 56. The IPG 14further comprises memory 68 and oscillator and clock circuitry 70coupled to the microcontroller 62. The microcontroller 62, incombination with the memory 68 and oscillator and clock circuit 70, thuscomprise a microprocessor system that carries out a program function inaccordance with a suitable program stored in the memory 68.Alternatively, for some applications, the function provided by themicroprocessor system may be carried out by a suitable state machine.

Thus, the microcontroller 62 generates the necessary control and statussignals, which allow the microcontroller 62 to control the operation ofthe IPG 14 in accordance with a selected operating program andstimulation parameters. In controlling the operation of the IPG 14, themicrocontroller 62 is able to individually generate a train of stimuluspulses at the electrodes 26 using the analog output circuitry 60, incombination with the control logic 52 and timer logic 56, therebyallowing each electrode 26 to be paired or grouped with other electrodes26, including the monopolar case electrode. In accordance withstimulation parameters stored within the memory 68, the microcontroller62 may control the polarity, amplitude, rate, pulse width and channelthrough which the current stimulus pulses are provided. Themicrocontroller 62 also facilitates the storage of electrical parameterdata (or other parameter data) measured by the monitoring circuitry 58within memory 68, and also provides any computational capability neededto analyze the raw electrical parameter data obtained from themonitoring circuitry 58 and compute numerical values from such rawelectrical parameter data.

Significantly, as will be described in further detail below, themicrocontroller 62 uses a set of arbitration rules to control themonitoring circuitry 58 to sense an electrical parameter within one ofthe channels k (effectively making it a sensing channel) that is notoccupied by the stimulation pulses. Alternatively, functions such as themanagement of stimulation pulses, timing information, and channelarbitration may be performed in a digital state machine, with themicrocontroller 62 having a supervisory role to manage information flow,e.g., sending stimulation parameters to the analog circuitry and/orconverting sampled analog data into a digital form, and thenpost-processing the digital data for storage or transmission to the RC16.

The IPG 14 further comprises an alternating current (AC) receiving coil72 for receiving programming data (e.g., the operating program and/orstimulation parameters) from the RC 16 (shown in FIG. 1) in anappropriate modulated carrier signal, and charging and forward telemetrycircuitry 74 for demodulating the carrier signal it receives through theAC receiving coil 72 to recover the programming data, which programmingdata is then stored within the memory 68, or within other memoryelements (not shown) distributed throughout the IPG 14.

The IPG 14 further comprises back telemetry circuitry 76 and analternating current (AC) transmission coil 78 for sending informationaldata sensed through the monitoring circuitry 58 to the RC 16. The backtelemetry features of the IPG 14 also allow its status to be checked.For example, when the RC 16 initiates a programming session with the IPG14, the capacity of the battery is telemetered, so that the externalprogrammer can calculate the estimated time to recharge. Any changesmade to the current stimulus parameters are confirmed through backtelemetry, thereby assuring that such changes have been correctlyreceived and implemented within the implant system. Moreover, uponinterrogation by the RC 16, all programmable settings stored within theIPG 14 may be uploaded to the RC 16. Significantly, the back telemetryfeatures allow raw or processed electrical parameter data (or otherparameter data) previously stored in the memory 68 to be downloaded fromthe IPG 14 to the RC 16, which information can be used to track thephysical activity of the patient.

The IPG 14 further comprises a rechargeable power source 80 and powercircuits 82 for providing the operating power to the IPG 14. Therechargeable power source 80 may, e.g., comprise a lithium-ion orlithium-ion polymer battery. The rechargeable battery 80 provides anunregulated voltage to the power circuits 82. The power circuits 82, inturn, generate the various voltages 84, some of which are regulated andsome of which are not, as needed by the various circuits located withinthe IPG 14. The rechargeable power source 80 is recharged usingrectified AC power (or DC power converted from AC power through othermeans, e.g., efficient AC-to-DC converter circuits, also known as“inverter circuits”) received by the AC receiving coil 72. To rechargethe power source 80, an external charger (not shown), which generatesthe AC magnetic field, is placed against, or otherwise adjacent, to thepatient's skin over the implanted IPG 14. The AC magnetic field emittedby the external charger induces AC currents in the AC receiving coil 72.The charging and forward telemetry circuitry 74 rectifies the AC currentto produce DC current, which is used to charge the power source 80.While the AC receiving coil 72 is described as being used for bothwirelessly receiving communications (e.g., programming and control data)and charging energy from the external device, it should be appreciatedthat the AC receiving coil 72 can be arranged as a dedicated chargingcoil, while another coil, such as coil 78, can be used forbi-directional telemetry.

It should be noted that the diagram of FIG. 6 is functional only, and isnot intended to be limiting. Those of skill in the art, given thedescriptions presented herein, should be able to readily fashionnumerous types of IPG circuits, or equivalent circuits, that carry outthe functions indicated and described, which functions include not onlyproducing a stimulus current or voltage on selected groups ofelectrodes, but also the ability to measure electrical parameter data atan activated or non-activated electrode.

Additional details concerning the above-described and other IPGs may befound in U.S. Pat. No. 6,516,227, U.S. Patent Publication No.2003/0139781, and U.S. patent application Ser. No. 11/138,632, entitled“Low Power Loss Current Digital-to-Analog Converter Used in anImplantable Pulse Generator,” which are expressly incorporated herein byreference. It should be noted that rather than an IPG, the SCS system 10may alternatively utilize an implantable receiver-stimulator (not shown)connected to leads 12. In this case, the power source, e.g., a battery,for powering the implanted receiver, as well as control circuitry tocommand the receiver-stimulator, will be contained in an externalcontroller inductively coupled to the receiver-stimulator via anelectromagnetic link. Data/power signals are transcutaneously coupledfrom a cable-connected transmission coil placed over the implantedreceiver-stimulator. The implanted receiver-stimulator receives thesignal and generates the stimulation in accordance with the controlsignals.

As briefly discussed above, the microcontroller 62 (or alternatively, adigital state machine), using a set of arbitration rules, controls themonitoring circuitry 58 to sense the electrical parameter within asensing channel that is not occupied by the stimulation pulses. That is,not only is the control circuitry configured for coordinating thestimulation timing channels, such that the stimulation pulses within therespective pulse trains do not temporally overlap any active phase ofeach other, the microcontroller 62 is configured for coordinating thestimulation timing channels and sensing timing channel, such thatelectrical parameter is sensed within time slots that do not temporallyoverlap any active phase of the stimulation pulses. For example, withreference to FIG. 7, stimulation pulses 102 are shown as beingtransmitted in stimulation channels 1, . . . N, while the electricalparameter 104 is shown as being sensed within the sensing channel duringtime slots that are not occupied by the stimulation pulses 102.

If the analog output circuitry 50 transmits pulses for the sensingfunction (e.g., when measuring impedance, field potentials, or evokedaction potentials), these pulses are preferably transmitted within thesensing timing channel between a selected group of electrodes 26, inwhich case, the monitoring circuitry 58 senses the electrical parameterin response to the transmission of the pulses. In some cases, e.g., whenmeasuring impedance and field potentials, the pulses used in the sensingfunction have relatively low charge-requirements (i.e., sub-thresholdpulses) to lessen the chances of patient perception during the sensingfunction. Also, as can be easily understood, the faster the sensingmeasurements are performed, the lower the probability that themeasurements will temporally overlap the stimulation pulses. With thisin mind, the monitoring circuitry 58 should be able to allow a fastsampling rate and require minimal charge injection during the sensingpulse, while still ensuring acceptable Signal-to-Noise ratio of themeasurement.

Referring to FIG. 8, one example of a technique for preventing thestimulation pulses transmitted respectively in two stimulation timingchannels and the sensing of the electrical parameter in the sensingtiming channel from overlapping each other will now be described. Thehorizontal axis is time, divided by increments of 1 millisecond (ms),while the vertical axis represents the amplitude of the electricalcurrent pulse, if any, applied to one of the sixteen electrodes E1-E16.

As there shown, a first train of bi-phasic stimulation pulses 110 (onlyone shown) is transmitted in Timing Channel 1, a second train ofbi-phasic stimulation pulses 112 (only one shown) is transmitted inTiming Channel 2, a third train of bi-phasic stimulation pulses 114(only one shown) is transmitted in Timing Channel 3, and a train ofphysiological samples 116 (only one shown) are taken in Timing Channel4. Each of the bi-phasic stimulation pulses includes an activestimulation phase (first phase) 118, a recharge phase (second phase)120, and an interphase (not shown) between the stimulation and rechargephases.

In the example, the initial stimulation pulse 110 is represented by a 4mA pulse transmitted between electrode E1 and electrode E3 within TimingChannel 1 at t=0 ms, with a cathodic (negative) current pulse of −4 mAappearing on electrode E1 and an anodic (positive) current pulse of +4mA appearing on electrode E3. Additionally, other stimulation parametersassociated with the bi-phasic stimulation pulse 110 are shown as beingset at a rate of 60 pulses per second (pps), and a pulse width of 300microseconds (μs).

The initial stimulation pulse 112 is represented by a 6 mA pulsetransmitted between electrodes E6, E7 and electrode E8 within TimingChannel 2 at t=2 ms, with cathodic (negative) current pulses of −4 mAand −2 mA appearing respectively on electrodes E6, E7, and an anodic(positive) current pulse of +6 mA appearing on electrode E8.Additionally, other stimulation parameters associated with the bi-phasicstimulation pulse 112 are shown as being set at a rate of 50 pulses persecond (pps), and a pulse width of 300 microseconds (μs).

The initial stimulation pulse 114 is represented by a 5 mA pulsetransmitted between electrode E8 and electrode E10 within Timing Channel3 at t=4 ms, with a cathodic (negative) current pulse of −5 mA appearingon electrode E8 and an anodic (positive) current pulse of +5 mAappearing on electrode E10. Additionally, other stimulation parametersassociated with the bi-phasic stimulation pulse 114 are shown as beingset at a rate of 60 pulses per second (pps), and a pulse width of 400microseconds (μs).

The initial sample 116 is shown as being taken at electrode E13 andreferenced at the IPG case at t=6 ms.

The particular electrodes that are used with each of the four timingchannels illustrated in FIG. 8 are only exemplary of many differentcombinations of electrode paring and electrode sharing that could beused. That is, any of the timing channels may be programmed to connectto any grouping of the electrodes, including the IPG case. While it istypical that only two electrodes be paired together for use by a givenchannel, as is the case with Timing Channels 1, 3, and 4 shown in FIG.8, it is to be noted that any number of electrodes may be grouped andused by a given timing channel. When more than two electrodes are usedwith a given channel, the sum of the current sourced from the anodic(positive) electrodes should be equal to the current sunk into thecathodic (negative) electrodes, as is the case with Timing Channel 2shown in FIG. 8.

The microcontroller 62 arbitrates the timing channels in accordance withthe following principles. Once a non-overlapping timing channel begins astimulation (or sample) pulse, the start of the pulses (or samples) fromany other overlapping timing channel is delayed until the ongoing firstphase of the pulse is completed and a hold-off period has beencompleted. If the start of two or more non-overlapping timing channelsare delayed by an ongoing pulse and hold-off period, the pending timingchannels are started in the order they would have occurred withoutarbitration. If two non-overlapping timing channels are scheduled tostart simultaneously, the lower number channel takes priority and startsfirst (i.e., Timing Channel 1 before Timing Channel 2, Timing Channel 2before Timing Channel 3, and Timing Channel 3 before Timing Channel 4).Because the sensing function is associated with Timing Channel 4 in thisexample, the sensing function has the lowest priority.

Current from any stimulus pulse (first phase) or active recharge (activesecond phase) is prevented from passing through any electrode undergoingpassive recharge. The delivery of an active first phase or active secondphase on any electrode takes precedence over all ongoing passiverecharge phases. Electrodes undergoing passive recharge have theirpassive recharge phases temporarily interrupted during the activephase(s). If the electrode is not part of the active phase, it remainsin a high impedance state (i.e., turned OFF) until the active phase iscompleted.

The above arbitration principles are illustrated in FIG. 9, which showsthe stimulation pulses and sample pulses associated with activeelectrodes. Recognizing that a channel comprises those electrodes thatprovide a stimulus current or measured sample of the same pulse width atthe same time, Timing Channel 1 comprises the group of electrodes E1,E3; Timing Channel 2 comprises the group of electrodes E6, E7, E8;Timing Channel 3 comprises the group of electrodes E8, E10; and TimingChannel 4 comprises the group of electrodes E9, IPG case. As shown inFIG. 8, the normal timing of the channel firings, without arbitration,would be as follows: Timing Channel 1 firing at time T1, Timing Channel2 firing at time T2, Timing Channel 3 firing at time T3, and TimingChannel 4 firing at time T4.

With arbitration, however, the timing of the channel firings is shown inFIG. 9. The first phase period 120 of Timing Channel 1 begins at timeT1. After the first phase period 120 is completed, the interphase period122 and hold-off period 126 of Timing Channel 1 begin, and aftercompletion of the interphase period 122, the second phase period 124begins. The second phase period 124 may be a fixed period of 5 ms, andthe hold-off period 126 may be a programmable delay, ranging from 1 msto 64 ms (in the illustrated embodiment, the hold-off period 126 isabout 3 ms). During the hold-off period 126, no other timing channelwill be active (i.e., will not stimulate or sense). Thus, at time T2,when Timing Channel 2 would normally fire, it is prevented from firing.Rather, Timing Channel 2 must wait a time period 126 until the hold-offperiod 124 concludes.

At the conclusion of the hold-off period 126, the first phase period 130of Timing Channel 2, which has priority over Timing Channel 3, begins.At this time, which is still during the second phase period 124 ofTiming Channel 1, the passive recharge taking place in Timing Channel 1is interrupted temporarily (e.g., for the duration of the active firstphase period 130 of Timing Channel 2). After the first phase period 130is completed, the interphase period 132 and hold-off period 136 ofTiming Channel 2 begin, and after completion of the interphase period132, the second phase period 134 begins. The second phase period 134 maybe a fixed period of 5 ms, and the hold-off period 136 is programmed tobe 15 ms. Neither Timing Channel 3 nor Timing Channel 4 is allowed tofire during the hold-off period 136.

As soon as the hold-off period 136 for Timing Channel 2 concludes,Timing Channels 3 and 4 are both past due for firing. The first phaseperiod 140 of Timing Channel 3, which has priority over Timing Channel4, begins. After the first phase period 140 is completed, the interphaseperiod 142 and hold-off period 146 of Timing Channel 3 begins, and aftercompletion of the interphase period 142, the second phase period 144 ofTiming Channel 3 begins. The second phase period 144 may be a fixedperiod of 5 ms, and the hold-off period 146 is programmed to be 3 ms.Timing Channel 4 is not allowed to fire during the hold-off period 146.

As soon as the hold-off period 146 for Timing Channel 3 concludes,Timing Channel 4 is past due for firing. The sensing period 150 ofTiming Channel 4 begins. At this time, which is still during the secondphase period 144 of Timing Channel 3, the passive recharge taking placein Timing Channel 3 is interrupted temporarily (e.g., for the durationof the active first phase period 140 of Timing Channel 3).Alternatively, the beginning of the sensing period 150 can be delayeduntil the second phase period 144 of Timing Channel 3 has beencompleted. After the sensing period 150 is completed, the hold-offperiod 156 of Timing Channel 4 begins. The hold-off period 156 isprogrammed to be 3 ms. Timing Channel 1 is not allowed to fire duringthe hold-off period 156. As soon as the hold-off period 156 for TimingChannel 4 concludes, the first phase period 120 of Timing Channel1begins, and the process repeats itself.

It should be noted that although the arbitration methodology has beendescribed as preventing any overlap between the stimulation and sensingchannels, there may be some applications in which partial overlapbetween the stimulation and sensing channels can be tolerated, and infact, preferred. For example, in Neural Response Imaging involving themeasurement of evoked potentials using cochlear implants, thestimulation and sensing channels may overlap as long as they aresynchronized with a certain delay. It can be appreciated that the sameconcept of timing management between the stimulation and the sensingactivities applies in this case. The key goal of the arbitrationmethodology is to ensure therapy is not modified by the sensing, whetherthis requires no-overlapping between the stimulation and sensingchannels (necessary for impedance or field potential measurements) orother forms of sensing where the timing relationship between thestimulation and sensing channels may be different.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. A neurostimulation system, comprising: analogoutput circuitry configured for conveying a train of electricalstimulation pulses within a stimulation timing channel between aselected group of an array of electrodes to stimulate neural tissue;monitoring circuitry configured for sensing an electrical parameterwithin a sensing timing channel using at least one of the electrodes;and control circuitry configured for coordinating the stimulation timingchannel and the sensing timing channel, such that the electricalparameter is sensed during the conveyance of the pulse train within timeslots that do not temporally overlap any active phase of the stimulationpulses.
 2. The neurostimulation system of claim 1, wherein the analogoutput circuitry is further configured for conveying another train ofelectrical stimulation pulses within another stimulation timing channelbetween another group of the electrodes in a manner that providesfurther therapy to the patient, wherein the control circuitry isconfigured for coordinating the stimulation timing channels and sensingtiming channel, such that the electrical parameter is sensed during theconveyance of the pulse trains within time slots that do not temporallyoverlap any active phase of the stimulation pulses.
 3. Theneurostimulation system of claim 2, wherein the control circuitry isfurther configured for coordinating the stimulation timing channels,such that the stimulation pulses within the respective pulse trains donot temporally overlap any active phase of each other.
 4. Theneurostimulation system of claim 2, further comprising a memoryconfigured for storing different stimulation parameters forindependently programming the stimulation timing channels.
 5. Theneurostimulation system of claim 1, wherein the analog output circuitryis configured for transmitting sub-threshold pulses within the sensingtiming channel between another group of electrodes, and the monitoringcircuitry is configured for sensing the electrical parameter in responseto the transmission of the sub-threshold pulses.
 6. The neurostimulationsystem of claim 1, wherein the electrical parameter is selected from thegroup consisting of an electrical impedance, a field potential, and anevoked action potential.
 7. The neurostimulation system of claim 1,further comprising the array of electrodes.
 8. The neurostimulationsystem of claim 1, wherein the control circuitry is configured forcoordinating the stimulation timing channel and the sensing timingchannel by providing a hold-off period after the active phase of thestimulation pulses, such that no electrical parameter is sensed duringthe hold-off period.
 9. The neurostimulation system of claim 1, furthercomprising telemetry circuitry configured for wirelessly transmittingthe sensed electrical parameter to an external programmer.
 10. Theneurostimulation system of claim 1, wherein the analog output circuitryis configured for conveying a continuous train of electrical stimulationpulses within the stimulation timing channel between the electrodes. 11.A method of providing therapy to a patient implanted with an array ofelectrodes using the neurostimulation system of claim 1, the methodcomprising: conveying from the analog output circuitry a train ofelectrical stimulation pulses within a stimulation timing channelbetween a group of the electrodes to stimulate neural tissue, therebyproviding therapy to the patient; and sensing by the monitoringcircuitry an electrical parameter within a sensing timing channel usingat least one of the electrodes, wherein the first stimulation timingchannel and sensing timing channel are coordinated by the controlcircuitry, such that the electrical parameter is sensed during theconveyance of the pulse train within time slots that do not temporallyoverlap any active phase of the stimulation pulses.
 12. The method ofclaim 11, further comprising conveying from the analog output circuitryanother train of electrical stimulation pulses within anotherstimulation timing channel between another group of the electrodes,thereby providing further therapy to the patient, wherein thestimulation timing channels and sensing timing channel are coordinatedby the control circuitry, such that the electrical parameter is sensedduring the conveyance of the pulse trains within time slots that do nottemporally overlap any active phase of the stimulation pulses.
 13. Themethod of claim 12, wherein the stimulation timing channels arecoordinated by the control circuitry, such that the stimulation pulseswithin the respective pulse trains do not temporally overlap any activephase of each other.
 14. The method of claim 12, further comprisingindependently programming the stimulation timing channels with differentstimulation parameters.
 15. The method of claim 11, further comprisingtransmitting from the analog output circuitry sub-threshold pulseswithin the sensing timing channel between another group of theelectrodes, wherein the electrical parameter is sensed by the monitoringcircuitry in response to the transmission of the sub-threshold pulses.16. The method of claim 11, wherein the electrical parameter is selectedfrom the group consisting of an electrical impedance, a field potential,and an evoked action potential.
 17. The method of claim 11, wherein theneural tissue is spinal cord tissue.
 18. The method of claim 11, furthercomprising implanting the array of electrodes within the patient. 19.The method of claim 11, wherein the coordination of the firststimulation timing channel and sensing timing channel comprisingproviding a hold-off period after the active phase of the stimulationpulses, such that no electrical parameter is sensed during the hold-offperiod.
 20. The method of claim 11, wherein the conveyance of the trainof electrical stimulation pulses within the stimulation timing channelbetween the electrodes provides continuous therapy to the patient.